In Wideband Code Division Multiple Access (WCDMA), Enhanced UpLink (EUL) has been available for some time for supporting transfer of larger quantities of information from a User Equipment (UE) to the Radio Base Station (RBS). The WCDMA EUL aims at scheduling traffic to times when the uplink interference situation is favorable, thereby utilizing air interface resources in as good a way as possible. The air interface load is measured by the noise rise, over the thermal level, a quantity denoted Rise over Thermal (RoT).
A “grant” is a permission for a UE to use certain uplink resources. The grants are thus the quantities signaled to the UE indicating what rate (actually power) it may use for its transmission. The UE is allowed to, but need not, use its complete grant. Relative grants are used to control the interference in neighbor cells. These can only decrease the current grant of the UE one step. It is stressed that there are only a discrete number of grant levels that can be used.
The task of the scheduler in a wireless communication system is to schedule EUL user traffic, to enhance user and cell capacity. At the same time the scheduler keeps track of the air interface cell load, avoiding over-scheduling that may cause cell instability and loss of coverage. The scheduler also keeps track of other available traffic, accounting for transport resources and hardware. The scheduler receives, measures and estimates quantities relevant for its scheduling operation, and transmits order to UEs, primarily in the form of granted power/bitrates.
The EUL utilizes a scheduler that aims at filling the load headroom of the air interface, so that the different user requests for bitrates are met. The air-interface load in WCDMA is typically determined in terms of the RoT, a quantity that is measured in the RBS. When evaluating scheduling decisions, the scheduler has to predict the load that results from the scheduled grants, to make sure that the scheduled load does not exceed the load thresholds for coverage and stability. This is complicated since the grant given to a UE does only express a limit on the UpLink (UL) power it is allowed to use, so the UE may use only a portion of its grant. The present scheduler makes typically a worst case analysis, assuming that all UEs use their grants at all times. Unfortunately, users do, at least from time to time, seem to have a relatively low utilization of grants.
There are also additional reasons why it is difficult to predict the load. This has to do with the fact that the link level channel conditions and the receiver performance has a strong impact on how large the load in terms of RoT becomes, for a given grant.
WCDMA has an inner loop power control aiming in adjusting the power levels. The inner loop power controller operates at a frequency of 1500 Hz. A Fast Congestion Control (FCC) functionality is often also available, where actions are taken in response to a declared power rush, normally by breaking at least a subset of the power control loops. This may be performed e.g. by sending a “down” command as long as an interference rush persists.
As a conclusion, the enhanced uplink of EUL operates by granting UEs the right to perform transmissions at a maximum rate. The grants are determined by the scheduler of the RBS that tries to fill the available load headroom, thereby optimizing the performance of EUL. The scheduler does this by prediction of the load caused by different scheduling decisions. The problem is unfortunately that the UEs does not have to use their grants. In particular smart phones tend to transmit small chunks of data and only need large grants occasionally. The result of this, also observed in the field, is a severe underutilization of EUL representing an unacceptable waste of resources.